In audio systems, due to existence of signal reflecting paths, interference of echoes is inevitable. Echoes in audio communications include electrical echoes and acoustic echoes, where electrical echoes are caused by signal energy reflection generated by impedance mismatch and acoustic echoes refer to voices from a speaker at a listening side are collected by a microphone and transmitted back to a speaking side, where acoustic echoes include direct acoustic echoes and indirect acoustic echoes. Direct acoustic echoes are voices from a speaker directly collected by a microphone, and indirect acoustic echoes are voices from a speaker collected by a microphone, where the voices pass different paths (e.g. a house or any objects in a house) and are reflected one or more times before being collected by the microphone. Echoes suffering channel latency are transmitted back to the speaking end and heard by a teller, so that causing interference on audios at the speaking end, which reduces clarity of the audios and affects quality of audio communications.
In order to cancel impacts on audio communications by echoes, in 1960s, Sondhi from the Bell Laboratory firstly presented that applying adaptive filters to realize cancellation of echoes. Referring to FIG. 1, a block diagram presenting a conventional system for adaptive echo cancellation is provided. Before a speaker 1 at a close end presents a downlink reference signal that is from a remote end, the downlink reference signal passes through an electrical echo path 2 and form an electrical echo; after the speaker 1 at the close end presents the downlink reference signal, the downlink reference signal is received by a microphone via an acoustic echo path 3 and form an acoustic echo. An adaptive filter 4 performs filtering to the downlink reference signal using an echo path model 5, and use a filtered output (namely, a cancellation signal) to cancel out the acoustic echo so that obtaining a residue signal (namely, an error signal), which is transmitted to the remote end. Simultaneously, an adaptive filtering algorithm 6 inside the adaptive filter 4 modifies parameters of the echo path model 5 based on the downlink reference signal and the error signal, so as to attenuate the remaining acoustic echo gradually.
In echo cancellation technologies, since an acoustic echo possesses characteristics that multi-path, long latency, slow attenuation, time-varying, non-linearity, etc., the adaptive filter 4 with good performance for Acoustic Echo Cancellation (AEC) is needed; especially, for hand-held devices with relatively severe non-linearity, the adaptive filter 4 with even better performance is required. Due to miniaturization of hand-held devices, comparing to regular speakers, micro speakers of the hand-held devices is much smaller in size. In order to achieve a required voice volume of hands-free communications, the micro speakers frequently operate in a non-linear domain, so that distortion becomes even more severe. Under this circumstance, the adaptive filter 4 provides a very unstable echo with very small loss, and the echo generally has no loss while facing a jump signal. Accordingly, providing a method and an apparatus for reducing an echo is required, which steadily provide a relatively high echo loss under a circumstance that a speaker suffers a relatively great distortion.